Field of the Invention
The present invention relates to a gas sensor for sensing a hydrocarbon gas, and more particularly, to a sensing electrode of the gas sensor.
Description of the Background Art
Gas sensors that sense a predetermined gas component of a measurement gas to determine its concentration come in various types such as semiconductor gas sensor, catalytic combustion gas sensor, oxygen-concentration difference sensing gas sensor, limiting current gas sensor, and mixed-potential gas sensor (for example, see Japanese Patent Nos. 3566089, 4405643, 4914447, and 5323752). Some of these gas sensors are obtained by providing electrodes containing a noble metal as its main constituent to a sensor element mainly composed of ceramic being a solid electrolyte such as zirconia.
Japanese Patent Nos. 3566089 and 5323752 disclose a limiting current gas sensor including a sensor element formed of a solid electrolyte, which includes electrodes made of Pt—Au alloy as pumping electrodes.
Japanese Patent No. 4405643 discloses a gas sensor provided with a thin layer containing Pt or Au as its main constituent to compensate for the adhesion between a solid electrolyte and an electrode made of metal oxide and gold.
Japanese Patent No. 4914447 discloses a mixed-potential gas sensor including a first electrode formed through application of a Pt—Au paste and a second electrode formed through application of a Pt paste and Au plating.
In response to more stringent regulations on exhaust gas, there have recently been increasing demands for a diagnosis of failure in the performance of cleaning unburned hydrocarbon in an exhaust emission control system (TWC: three-way catalyst) of a gasoline engine and a diagnosis of failure in the performance of cleaning unburned hydrocarbon in an exhaust emission control system (DOC: diesel oxidation catalyst) of a diesel engine. These diagnoses require a gas sensor capable of sensing an unburned hydrocarbon gas and identifying its concentration.
The inventors of the present invention have made intensive studies to find out that in a sensing electrode made of Pt—Au alloy having an increased Au abundance ratio, the catalytic activation against a hydrocarbon gas is disabled, inducing a mixed potential having correlation with the concentration of the hydrocarbon gas. Such finding has led the inventors to a gas sensor capable of sensing a hydrocarbon gas with high sensitivity.
In the invention disclosed in Japanese Patent No. 4914447, the concentration of a gas component is determined on the premise that both of the first electrode and the second electrode have catalytic activation, although there may be a slight difference. In
Japanese Patent No. 4405643, the relationship between the alloy composition of the electrode and detection sensitivity is not clear.
Japanese Patent No. 4405643 discloses that a pumping electrode for a limiting current gas sensor is made of Pt—Au alloy such that an Au abundance ratio is 0.01 or more and 0.3 or less, thereby increasing the selective decomposition ability for oxygen in the pumping electrode. Japanese Patent No. 4405643 also discloses that an Au abundance ratio exceeding 0.3 is not preferable because such a ratio increases electrode impedance. Japanese Patent No. 5323752, however, discloses or suggests nothing about a mixed-potential gas sensor (needless to say, about its sensing electrode as sell).